Glass containers for use as packages for beverage products, such as beer, refreshing drinks, milk, yogurt, and coffee, have conventionally been collected from the market after use, washed, and then reused. As such reusable glass containers are used repeatedly, their appearances are impaired more and more because the containers suffer abrasions on the surface thereof during the filling processes and subsequent shipping due to contacts with each other or with metallic parts of machines and apparatuses. The impaired appearance quality of the packages results in decreased commercial values of the beverages in the containers. The above has been the state of things in the reusable glass containers.
Under these circumstances, an abrasion-preventing agent which serves to prevent glass containers from suffering abrasions, a concealing agent which is coated over abrasions on glass container surfaces to conceal the abrasions, and an abrasion-preventive concealing agent which has both abrasion-preventing effect and abrasion-concealing effect (hereinafter referred to as an abrasion-concealing agent) have been devised and several agents of such types have so far been proposed. Such an abrasion-concealing agent should simultaneously satisfy some requirements such as the following:
(1) A film obtained from an abrasion-concealing agent should have good abrasion-concealing properties;
(2) The film should have a high film strength under abrasion-occurring conditions;
(3) The film should have good water resistance;
(4) The film should be free of surface tack;
(5) An abrasion-concealing agent should be easily coated and able to form, at ordinary temperature, a film having the required properties;
(6) An abrasion-concealing agent should be non-toxic;
(7) The film should be easily and completely peeled off by washing with an aqueous alkaline solution; and
(8) An abrasion-concealing agent should not cause damage to a label attached to the glass container.
The above requirements are explained in detail below.
The film must, of course, have good abrasion-concealing properties first. In addition, the film is required to have a high film strength sufficient to withstand conditions under which abrasions occur due to contacts with machines and plastic cases during the beverage-filling process and packing operations after film formation and during subsequent shipping. The water resistance of the film must be such that immersion of a glass container in water does not result in blushing or peeling of the film, because glass containers filled with beer or refreshing drinks often undergo water condensation after cooled in refrigerators or showcases and then taken out therefrom and are also often immersed in cold water. The tack-free properties of the film surface are required to give good hand feeling in handling and to avoid fouling of the glass container by adhesion of dust, etc. Easiness of coating and the ability to form at ordinary temperature a film having the necessary properties are required because the flammability is diminished, the safety of the working atmosphere is ensured if the solvent vapor is non-toxic, the coating equipment, etc., are not expensive, and coating after filling can be conducted without changing the quality of the contents. Further, the abrasion-concealing agent and the film obtained therefrom should be non-toxic and odorless because the glass container is to contain a beverage.
Furthermore, the reasons why the film is required to be peeled off with an aqueous alkaline solution are as follows. Even if the film is a permanent coating which is tenaciously adhering to the glass container, and is not peelable with alkali, it is difficult to prevent the permanent coating itself from suffering abrasions and it is impossible to prevent the coating film from suffering damages due to washing process, which is repeated for repeated use of the glass container. Therefore, it is, after all, rational that the film be easily peeled off when the glass container is washed for repeated use and a new film be formed by coating treatment. Such a method is excellent from the standpoint of appearance quality as well.
Still more important for the abrasion-concealing agent than the above requirements is not to damage a label attached to the glass container. Coating of the abrasion-concealing agent on a glass container may be conducted either before or after the labeling. If the label is damaged by the abrasion-concealing agent or by the coating treatment or damaged as a result of the coating, not only such a label damage impairs appearance quality more severely than abrasions on the glass container do, but the function of the design or trademark printed on the label is impaired.
Abrasion-concealing agents which have so far been proposed include, for example, emulsions obtained by emulsifying liquid paraffins, natural waxes such as carnauba wax, animal or vegetable fats and oils, glycerin, or the like, these being used alone or in combination, with a surfactant such as a fatty acid ester or a fatty acid salt (JP-A-59-102973, JP-A-59-111947, and JP-A-59-145259). (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) However, the abrasion-concealing agents using liquid paraffins or ordinary temperature-liquid fats or oils are extremely defective in that the coating films obtained therefrom are apt to be tacky and the water resistance of the films are so poor that immersion thereof in water results in formation of an oil film on the water surface. On the other hand, the abrasion-concealing agent using waxes having high melting point are defective in that they are poor in the effect of concealing abrasions, and in the process of washing with an aqueous alkaline solution, coating film pieces that have peeled off from the glass container float on the surface of the cleaning fluid in the form of a scum and re-adhere to the glass container. Further, the agents using surfactants have poor water resistance, so that the coating films disadvantageously peel off easily upon immersion in water.
An abrasion-concealing agent which is an aqueous emulsion or organic solvent solution of an organic polymer as a main component has also been proposed, but the agent cannot simultaneously satisfy both of the property of forming a film at ordinary temperature and the property of being peeled off with an aqueous alkaline solution.
On the other hand, a composition comprising a polyorganosiloxane having a silicon-bonded methyl, phenyl, and alkoxy group and a curing catalyst therefor (JP-A-55-56040) and a composition obtained by introducing a surfactant in the above composition (JP-A-56-23444) have been disclosed. The two compositions which use as a film-forming component a polyorganosiloxane having a methyl group and a phenyl group show an improved abrasion-concealing effect because the cured film has a heightened refractive index mainly due to the introduction of phenyl group so as to have the same refractive index as that of glass. However, there are cases where due to the introduction of phenyl group, the film is made poor in the property of being peeled off by washing with an aqueous alkaline solution. Moreover, since the above compositions are in the form of an alcohol solution, especially an ethanol solution, from the safety and health standpoint, the compositions tend to attack the resin coating layer on the label surface, resulting in a problem that the label damage impairs appearance quality. In addition, since such abrasion-concealing agents have flash points as low as around 13.degree. C. due to the ethanol, they have a disadvantage that care must be taken in storage and coating to ensure the safety of the working atmosphere.